Why does airspeed change when I adjust the prop?

Mxsmanic wrote:
OK, but why does the airspeed drop? In a car, you use the highest
gears (coarsest pitch, hence lowest prop RPM) for high-speed cruise.

Because the RPM drops. Ceteris paribus, the same amount of fuel/air mixture
enters each cylinder with each intake stroke, and each cylinder imparts the
same amount of torque with each power stroke, but because of the lower rpm,
you have less power strokes per time, thus, the engine yields less power.
If you reduce the RPMs by 20%, c.p.* the engine power will drop by 20%,
too. Hence, you loose airspeed.

And the gearbox-metaphor is not very well suited because most cars don't
have a contiously-variable transmission but rather distinct gears, also,
you don't specify the desired engine rpm but rather the desired gear ratio.
That is not the case with the constant-speed prop, you select a desired
rpm, and the prop governor adjusts the load on the engine (by varying the
prop pitch) to maintain that rpm, regardless of the actual power output of
the engine (within the limits of the prop's abilities, of course).

Anno.

*) and that's a pretty strong c.p.

Anno v. Heimburg writes:

And the gearbox-metaphor is not very well suited because most cars don't
have a contiously-variable transmission ...

FWIW, I did have a scooter with this type of transmission. It was a
bit bizarre, because if you opened the throttle wide open from a
standing start, the engine would speed up to its optimal RPM, and then
the transmission would continuously change ratios to keep the engine
at the same RPM as the scooter accelerated. You'd hear a constant
whirr from the engine, even though you were accelerating very rapidly.
It also provided good acceleration because of this, since the
(relatively small) engine was held at a single optimum speed.

That is not the case with the constant-speed prop, you select a desired
rpm, and the prop governor adjusts the load on the engine (by varying the
prop pitch) to maintain that rpm, regardless of the actual power output of
the engine (within the limits of the prop's abilities, of course).

So what do you gain with the prop adjustment? Just a reduction in
wear and tear and/or noise?

--
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On Tue, 16 Jan 2007 10:29:50 -0800, Mxsmanic wrote
(in article ):



So what do you gain with the prop adjustment? Just a reduction in
wear and tear and/or noise?

No, you get a lot more than that. If you had the manual for a Baron you would
see. But let us look at the Cessna 206H, a single engine plane with a
constant speed prop.

At 8000 feet, with 22 inches MP and 2500 RPM, you generate 69BPH and have an
airspeed of 139 KTAS and burn 15.6 GPH. At 2100 RPM you generate only 57 BPH
and have an airspeed of 126 KTAS and burn 13.2 GPH. This is interesting, in
that you get exactly these same numbers (57 BPH, 126 KTAS, and 13.2 GPH) if
you have 2200 RPM but only 21 inches MP. And it is only slightly different at
2300 RPM and 20 inches MP.

Obviously, a reduction in either MP or RPM reduces your horsepower, airspeed,
and fuel consumption.

If you were in a fixed prop Cessna 172, you would have your throttle full
forward on takeoff in order to generate maximum horsepower. You want to get
away from the ground as quickly as possible, both for safety and noise
abatement (the sound of crumpling metal annoys the neighbors). But because
the prop has a fixed pitch, you cannot increase RPM. You cannot increase MP
because you already gave it full throttle. As the airplane pitches up the
prop will slow down. So you no longer have full power. If only you could
reduce the pitch of the prop to keep it at 2700 rpm. Well, in a CSP plane,
you can do exactly that. A constant speed prop is a performance enhancement.
It allows you to use all the power the engine has, at least until you have
climbed high enough that you need to turbocharge the engine, anyway.